Low‐Temperature Sputtered Ultralow‐Loss Silicon Nitride for Hybrid Photonic Integration

Abstract

Silicon‐nitride‐on‐insulator (Si3N4) photonic circuits have seen tremendous advances in many applications, such as on‐chip frequency combs, Lidar, telecommunications, and spectroscopy. So far, the best film quality has been achieved with low pressure chemical vapor deposition (LPCVD) and high‐temperature annealing (1200°C). However, high processing temperatures pose challenges to the cointegration of Si3N4 with pre‐processed silicon electronic and photonic devices, lithium niobate on insulator (LNOI), and Ge‐on‐Si photodiodes. This limits LPCVD as a front‐end‐of‐line process. Here, ultralow‐loss Si3N4 photonics based on room‐temperature reactive sputtering is demonstrated. Propagation losses as low as 5.4 dB m−1 after 400°C annealing and 3.5 dB m−1 after 800°C annealing are achieved, enabling ring resonators with highest optical quality factors of > 10 million and an average quality factor of 7.5 million. To the best of the knowledge, these are the lowest propagation losses achieved with low temperature Si3N4. This ultralow loss enables the generation of microresonator soliton frequency combs with threshold powers of 1.1 mW. The introduced sputtering process offers full complementary metal oxide semiconductor (CMOS) compatibility with front‐end silicon electronics and photonics. This could enable hybrid 3D integration of low loss waveguides with integrated lasers and lithium niobate on insulator.

Sub-ppm Nanomechanical Absorption Spectroscopy of Silicon Nitride

“…As is evident in Figure , the Si 3 N 4 extinction coefficients we infer from NAS, κ ∼ 0.1–1 ppm (corresponding to a propagation loss of ∼0.1–1 dB/cm), are on par with lower bounds from waveguide direct transmission measurements (DAS), − roughly 10-fold higher than lower bounds from waveguide CAS, ,− and roughly 10-fold lower than obtained by methods applied to Si 3 N 4 membranes, such as photothermal common path interferometry (PCI) and membrane-in-the-middle (MIM) CAS. ,− Possible reasons for the latter discrepancies include the sensitivity of waveguide-based DAS/CAS to scattering loss (e.g., due to sidewall roughness) and the sensitivity of membrane-based DAS/CAS to diffraction loss (e.g., tip–tilt misalignment in MIM systems) and surface contamination. We also re-emphasize that NAS is sensitive to uncertainties in material properties entering eq ; however, these uncertainties do not appear to account for the 10-fold discrepancy with MIM and PCI measurements.…”

“…Compilation of extinction coefficient measurements for silicon nitride. Surveyed methods include direct (single-pass) absorption spectroscopy (DAS) in thick films and high-confinement (>50%) waveguides, − cavity-enhanced absorption spectroscopy (CAS) with waveguide microresonators ,− and membrane-in-the-middle (MIM) cavity optomechanical systems, ,− photothermal common path interferometry (PCI), ellipsometry, and nanomechanical frequency shift absorption spectroscopy (NAS, this work). The gray region is the rough cutoff wavelength for two-photon absorption.…”

Anneal-free ultra-low loss silicon nitride integrated photonics

“…We see that this satisfies the >50% cladding absorption definition of thin waveguides that we introduce in the section “Anneal-free fabrication process and waveguide design”. Previously reported work on thick core low-temperature nitrides using deuterated processes 56 , 58 , 61 as well as sputtering 62 did not demonstrate low absorption loss for their upper claddings and hence ultra-low loss thin nitride devices were not achieved. We note that we could reduce the nitride core thickness further to achieve losses closer to the cladding absorption limit; however, this would come at the expense of increased critical bend radius and device area.…”

“…The different works compared include inductively coupled plasma-plasma enhanced chemical vapor deposition (ICP-PECVD) processes using deuterated silane precursors like (I) This work, (II) Y. Xie et al 58 , (III) J. Chiles et al 56 , and (IV) X.X. Chia et al 61 —which also uses Si-rich SiN; Sputtering such as (V) A. Frigg et al 83 , (VI) S. Zhang et al 62 ; plasma enhanced chemical vapor deposition (PECVD) in conjunction with chemical–mechanical polishing (CMP) (VII) X. Ji et al 90 ; pulsed laser deposition—(VIII) N. Golshani et al 91 ; And low-pressure chemical vapor deposition (LPCVD) together with annealing, such as (IX) Z. Ye et al 92 , (X) X. Ji et al 52 , (XI) K. Liu et al 42 , (XII) J. Liu et al 76 — which uses a Damascene process too, and (XIII) W. Sun et al 93 …”

“…The first Si-rich deuterated thick nitride waveguides demonstrated losses of 150 dB m −1 and resonator intrinsic Q of 1.32 × 10 5 with a 350 °C process 54 , 55 , 61 . Hydrogen-free low-temperature sputtering has also been employed, combined with 300 °C deposited upper cladding, to achieve 32 dB m −1 losses and 1.1 million intrinsic Q in 750 nm core waveguides 62 . After 400 °C annealing these achieved 5.4 dB m −1 loss and 6.2 million intrinsic Q.…”